1. Field of the Invention
This invention relates to high strength, high temperature corrosion and erosion resistant structures and their processes of preparation. More particularly, this invention relates to cost effective, low heat capacity, lightweight composite structures with tailored thermal conductivity which are capable of sustaining heavy loads and withstanding highly corrosive and erosive environments at greatly elevated temperatures and pressures for hours or days or even more.
2. Description of the Prior Art
Previously considerable difficulty had been experienced in providing efficient structures which retain their structural integrity at temperatures in excess of 1,500 degrees centigrade while sustaining substantial structural loads in harsh chemical environments. Previous solutions have included that proposed in Tuffias et al U.S. Pat. No. 4,917,968, wherein a structure composed of metallurgically bonded layers of a platinum group metal such as ductile iridium and a refractory metal such as ductile rhenium possesses both the high temperature corrosion resistance and the high temperature structural strength for high stress, high temperature applications. When additional strength was required, according to previous teachings, it was generally believed necessary to increase the thickness of the structure, particularly the thickness of the refractory metal layer. Increasing the strength of the structure by increasing its thickness increased the weight, the heat capacity, the cost and the difficulty of manufacturing it. These limitations are undesirable in a number of fields including, for example, that of rocketry. Rocket thrust chamber assemblies, for example, could be improved by the availability of lightweight, low cost, high strength, low heat capacity refractory materials. Further, the ability to selectively tailor the strength, heat capacity and thermal conductivity of various parts of the rocket thrust chamber assemblies would permit greater efficiencies and design flexibility.
As described, for example, in Tuffias et al U.S. Pat. No. 4,917,968, chemical vapor deposition procedures are utilized to form successive layers of a structure under conditions such that the layers are bonded together by an interlayer which is an admixture of the two adjacent layers. A first corrosion resistant layer is provided which is capable of withstanding highly corrosive environments at temperatures in excess of 1,500 degrees centigrade, and preferably, in excess of 2,000 or 2,200 degrees centigrade. The corrosion resistant layer is bonded through an interlayer to a high strength layer. The high strength layer exhibits tensile strengths in excess of 5,000, and preferably, 15,000 pounds per square inch at temperatures in excess of 1,500, and preferably, in excess of 2,000 degrees centigrade. In general, the metallic layers in such structures are composed of platinum group metals such as, for example, iridium, and refractory materials, preferably metals such as, for example, rhenium. The hot sides or normally interior sides of the structures may, if desired, be coated with high temperature ceramic materials such as, for example, hafnium dioxide or zirconium dioxide, to improve their corrosion resistance.
When rocket thrusters are operated at the optimum oxidizer/fuel ratio, the temperature of the combustion products may be as high as 2,000 to 3,000 degrees centigrade. This is the condition at which the highest efficiency is achieved. Thrusters must be cycled on and off many times over their useful lives. The thrusters may cool between cycles to very low temperatures. Further, thrusters must withstand the forces imposed on space vehicles during launch and in use. The materials from which thrusters are constructed must withstand both structural and thermal shock. Brittle materials tend to fail rapidly because of an inability to withstand this shock. Premature failure because of cracking under shock or thermochemical failure greatly limits the useful life and reliability of the thruster. Very few materials or material combinations are capable of withstanding the shock loads, the structural loads, and the catastrophically corrosive conditions which are imposed on, for example, a rocket thrust chamber assembly. The metallurgically bonded Iridium-Rhenium structure described in Tuffias et al U. S. Pat. No. 4,917,968 is capable of fulfilling these requirements except that increasing the mass of the structure to accommodate particularly robust applications produces some undesired results.
Increasing the volume of the metallic parts of the rocket motor in an attempt to increase strength and durability is generally self defeating. The cost of operating a spacecraft with heavier components escalates rapidly, and efficiency degenerates rapidly to unacceptable levels. The cost and difficulty of manufacturing high quality parts escalates quickly as the volume and weight of metal increases, particularly where the structure has a high degree of concavity. When operating out of the atmosphere, rocket motors are cooled by radiation. Under these conditions high heat capacity structures are undesirable because the heat retained in the structure of the rocket motor flows back into the adjacent structures and electronics, including the fuel system components, especially during shut down. This may cause the fuel system or other components to fail or, at the worst, it may cause the fuel to ignite outside of the rocket motor chamber with catastrophic results. In general, the heat in the rocket motor should be dissipated from the skirt region by radiation. Preferably, thermal conductivity between the skirt region and the rest of the rocket motor should be enhanced. These objectives are generally incompatible with increasing the strength of the motor by increasing the thickness of its refractory metal walls.
Propulsion systems incorporating thrust chamber assemblies are used in space for the maneuvering of satellites and otherwise, in air for aircraft intercept systems, ground based intercept systems, and the like. Improving the efficiency of the thruster and extending its life provides a substantial number of new alternatives in these and other applications. The same thrust can be obtained with less fuel and thus less weight. The savings in weight can be distributed between additional fuel and additional payload. Extending the life of the thruster and the number of cycles which it can withstand prolongs the useful life of the satellite or vehicle upon which it is mounted. Increasing the effective fuel capacity of the vehicle also extends its useful life.
Many applications exist outside of the space field for lightweight, relatively inexpensive structures which are capable of withstanding shock, and high structural loads in highly corrosive high temperature environments. The absence of such structures limits or precludes the use of some reactions in the chemical process industry. Such structures find application in propulsion systems and prime movers other than rockets. Other fields such as nuclear, metallurgical, waste disposal, emissions reduction, and the like, also require such structures to optimize or make possible various operations.
Chemical vapor deposition procedures are known and have been used for the better part of a century for forming various coatings. In general it is a method of plating on an atom by atom basis in which a gaseous compound, or compounds, of the material to be deposited is flowed over or through a heated substrate, resulting in the thermal decomposition or reduction of the compounds and the subsequent deposition of the material onto an exterior or interior surface of the heated substrate. The parameters which must be controlled for successful reliable operation include the choice of gaseous compounds, the concentration of the compounds in the gas, the gas flow rate, the gas pressure, the nature of the substrate material, the geometry of the substrate, the temperature of the substrate, and the geometry of the reaction chamber. The nature of the deposit may be controlled by controlling these parameters. The crystal form may, for example, sometimes be changed by changing the gaseous compounds. The degree of adherence to the substrate may be controlled, for example, by adjusting the temperature of the substrate. The deposits formed by chemical vapor deposition are generally very pure. Materials may be codeposited depending upon the composition of the gas which is supplied to the reactor. Both metallic and non-metallic materials may be deposited using these techniques and they may even be codeposited. In general, the preferred method of construction of hollow structures is that whereby successive layers are formed on a mandrel which is later removed, for example, by chemical leaching. The structure is thus formed inside out, thereby reflecting the outer surface of the mandrel.
For the sake of convenience in describing and defining the process and the structure, the cross-section of the structure has been referred to as comprising various layers of materials. If desired, however, it is possible with chemical vapor deposition techniques to provide a continuous variation in the composition of the structure from one pure material at one outer surface to another pure material at the opposed outer surface. Also, the variation in the composition need not be continuous. If ceramics are used on one outer surface it is possible to provide such a continuous variation in composition across three or more materials, not all of which are metallic. Such graded deposits have no discontinuity and thus no stress concentration due to a mismatch of thermal expansion rates. Graded deposits are produced by varying the composition of the reactive gas as the deposit builds up. The description and definition of the process and structure is intended to include such graded deposits.
In general the chemical vapor deposition operation which is utilized to form the structure is controlled so that the first deposit is not necessarily chemically or metallurgically bonded to the mandrel. Subsequent deposits are bonded very tightly to one another through interlayers. Adjustment of the temperature of the substrate and the period of time during which the substrate is subjected to elevated temperatures permit control of the extent and nature of the bond. The preferred temperatures should be determined for each specific situation, and may vary considerably from one application to another.
Less preferred alternative coating methods may be used, if desired, in the formation of part or all of the structures described. Such methods include, but are not limited to, for example, electrolytic, electroless, physical vapor deposition (PVD), plasma spray techniques, and the like.
The requirement that the structure be possessed of shock resistance and high strength at very high temperatures severely limits the choice of refractory materials for the high strength barrier or layer. Materials which have tensile strengths in excess of 5,000 pounds per square inch at 2,000 degrees centigrade include, for example, carbon-carbon, tungsten, Ta-10W, Mo-50Re, rhenium, and thoriated tungsten (W-1ThO.sub.2). Of these-materials, only carbon-carbon, W-1ThO.sub.2, rhenium and Mo-50Re have tensile strengths approximately in excess of 10,000 pounds per square inch at 2,000 degrees centigrade. Alloys, admixtures, and composites of these materials with each other and with other materials are also suitable for use as the structural barrier.
The requirement that the corrosion resistant layer or barrier withstand shock and catastrophically corrosive conditions at temperatures in excess of 1,500 and preferably in excess of 2,000 or 2,200 degrees centigrade for several hours severely limits the choice of materials for this layer or barrier. The platinum group metals and their alloys are candidates for this barrier. Iridium and rhodium and their alloys with each other and with, for example, platinum, rhenium and osmium, are suitable candidates. The addition, for example, of approximately 30 percent platinum or rhodium to iridium reduces the melting point of the alloy to approximately 2,200 degrees centigrade but substantially increases the resistance of the material to oxidation.
Iridium structures, and certain iridium alloy structures, which have been carefully prepared by chemical vapor deposition procedures are inherently ductile. This permits the structural layer to deform somewhat under applied loads without destroying the integrity of the corrosion barrier.
Where it is desired to protect exposed surfaces of the structure, and particularly the exposed surface of the corrosion barrier, with a ceramic material there are a number of possible suitable materials. Hafnium dioxide and zirconium dioxide are very suitable, particularly when stabilized with an inversion inhibitor. Yttria, for example, has been used to inhibit the crystalline inversion of hafnium dioxide. Crystalline inversion occurs with hafnium dioxide at approximately 1,600 degrees centigrade and causes cracking and spalling of hafnia coatings. Other borides, carbides and nitrides from which suitable materials may be selected include, for example, hafnium, tantalum, zirconium, tungsten, silicon, and boron carbides, tantalum, hafnium, boron, zirconium, titanium, and niobium nitrides, and hafnium, zirconium, tantalum, niobium, and titanium borides. The ceramic layer, if used, is very thin and is not intended to provide the primary corrosion protection for the structural layer. The purpose of the ceramic barrier is to reduce the recession rate of the primary corrosion barrier. The ceramic and/or primary corrosion barrier may be applied to all of the exposed surfaces of the structure if desired. The thickness of the ceramic layer is generally three mils or less. This may be varied as conditions may require.
Carbon fiber reinforced carbon matrix composites are known to be lightweight, high strength, refractory, structural materials whose strength increases with temperature, even at temperatures well in excess of 2,000 degrees Centigrade. Well known limitations on the use of carbon-carbon composites include the facts that they rapidly oxidize catastrophically at temperatures well below 1,000 degrees Centigrade, and generally exhibit some degree of porosity. These composites must, therefore, be protected from oxidation, and are generally not satisfactory where an impermeable structure is required. In the preparation of carbon-carbon composite structures, the carbon fibers and filaments are generally formed into a construct or preform of the desired configuration by winding, weaving, knitting, braiding, or wrapping over a suitably formed support or mandrel. The carbon matrix is generally formed by infiltrating the preform with resin, which is then pyrolyzed, or by chemical vapor deposition techniques, or both, whereby carbon is deposited in the construct. The characteristics of the completed composite are determined by the nature of the starting materials and the processing. The thermal and mechanical properties of carbon-carbon composites are highly tailorable.